1) Field of the Invention
The invention relates to microwave-assisted heating, and more particularly, to systems for microwave processing of a plurality of laboratory samples.
2) Description of the Prior Art
Most chemical reactions either require or benefit from the application of heat. Developments have provided for the use of microwave heating instead of typical Bunsen burners or “hot plates”. The use of microwave energy is known to be quite appropriate for many chemical reactions. Microwave heating represents the use of radiation energy at wavelengths residing in the electromagnetic spectrum, or between the far infrared and the radio frequency (from about one millimeter (mm) to about 30 centimeters (cm) wavelengths, or with corresponding frequencies in the range of about 1 to 300 gigahertz (GHz)). The exact upper and lower limits defining “microwave” radiations are somewhat arbitrary.
Microwave radiation is widely used in several fields like spectroscopy, communication, navigation, medicine, and heating. Substances that respond quite well by increasing their temperature levels when under microwave radiation usually have a high dielectric absorption. The use of microwave heating in laboratories is known to people skilled in the art and is often referred to as “microwave assisted” chemistry. A number of laboratory microwave heating devices are thus commercially available. These microwave heating devices typically use a magnetron as the microwave source, a waveguide (usually hollow circular or rectangular metal tube of uniform cross section) to guide the microwaves, and a resonator (sometimes also referred to as the “cavity”) into which the microwaves are directed to heat a sample. The microwave source can also be a Klystron, traveling wave tubes, oscillators, and certain semiconductor devices. Most devices use magnetrons, however, as these are simple and economical. One disadvantage of magnetrons is that the control of radiation power directed towards a specific sample inserted inside a resonator is somewhat complex. One known method of controlling the radiation of the magnetron is to run it at its designated constant power while turning it on and off on a cyclical basis in order to have a certain temperature control of the sample(s) located inside separate containers or loads made of a microwave transparent material such as some types of glass, plastic or ceramic. Usually, for convenience, only one load is monitored within the group of loads each containing a sample, the remaining loads estimated to behave somewhat similarly. This leads to large amounts of uncertainty as to the evolution of reactions inside other loads, since even when a “stirring” device can produce quite uniform radiation inside the cavity of a microwave heater, several other factors, such as the presence of samples and sample containers in the microwave oven, can also change the interference pattern within the cavity and thus affect the energy distribution inside the cavity.
Accordingly, when multiple samples are to be treated under one microwave source, the treatment should be uniform and controllable. Hence, there is a need to provide for the ability to vary the radiation power levels sent to each sample using a limited number of microwave sources in order to maintain low costs and high efficiency. There is also a need to be able to precisely know and control the temperature or amount of radiation power sent to each individual sample.